Population dynamics and secondary production of the cockle Cerastoderma edule: A comparison between Merja Zerga (Moroccan Atlantic Coast) and Arcachon Bay (French Atlantic Coast)
Cockle (Cerastoderma edule) population dynamics were studied at the southern limit of the distribution of this marine bivalve in Merja Zerga, Morocco. Parameters such as growth, mortality, and production were compared with those of a population at Arcachon Bay (France) a site in the center of the cockle's range. At each sampling period between two and three cohorts were simultaneously observed at each site and the average total abundance was usually higher at Merja Zerga. Recruitment occurred at both sites in spring when temperature rose above 19 °C, independently of the month. In Merja Zerga, winter recruitment was also observed at one occasion, following high sediment disturbance. The first year (2005–06) at Merja Zerga, the mortality rate was close to nil for juveniles and was Z = 1.5 yr− 1 for adults, providing a high production (64 g dry weight m− 2 yr− 1). At Arcachon during the same period, the juvenile mortality rate was Z = 10.9 yr− 1, the adult mortality rate was 3.4 yr− 1 and production was 26 gDW m−2 yr− 1. The second year (2006–07), mortality after recruitment was much higher (Z = 8.6 yr− 1, for juveniles) and similar to what was observed at Arcachon (Z = 8.4 yr− 1). Mortality rate of adults was higher at Merja Zerga (Z = 3.0 yr− 1) than at Arcachon (Z = 1.5 yr− 1). Production was lower at Arcachon than at Merja Zerga although growth performances were higher at Arcachon. The higher growth performance at Arcachon (Φ′ = 3.3) was mainly due to high asymptotic length (L∞ = 38 mm) and was related to low intraspecific competition compared to Merja Zerga where cockle abundance was higher (Φ′ = 3.1, L∞ = 31 mm). P/B was low in both sites and slightly higher at Arcachon (1.1–1.5 against 1.0–1.1 yr− 1). At Arcachon, recruitment was correlated with temperature, a peak occurring when temperature rose above 19 °C (June–July). At Merja Zerga, recruitment was already 2–3 months earlier but was not significantly correlated to temperature. This study showed that population dynamics of cockles at the southern limit of this distribution fell in the range of what was observed elsewhere in the North-Eastern Atlantic coast. Most factors that were involved in population regulation (intraspecific competition, predation and sediment dynamics) were not strictly dependent on latitude. The direct role of temperature (latitude dependent factor) was not obvious. Variation in temperature could explain the recruitment delay between Arcachon and Merja Zerga and the low maximum shell length at Merja Zerga.
(Journal of Sea Research (JSR). vol. 63, n° 1385-1101, pp. 191-201, 01/04/2010)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Oxygen and organic carbon fluxes in sediments of the Bay of Biscay
(Deep Sea Research Part I: Oceanographic Research Papers. vol. 57, n° 0967-0637, pp. 528-540, 01/04/2010)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Cadmium-induced genotoxicity in zebrafish at environmentally relevant dose
(Ecotoxicology and Environmental Safety. vol. 73, n° 0147-6513, pp. 312-319, 01/03/2010)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, LMBA, UBS, UBO EPE, CNRS
The first estimation of Fleuve Manche palaeoriver discharge during the last deglaciation: Evidence for Fennoscandian ice sheet meltwater flow in the English Channel ca 20–18 ka ago
The Fleuve Manche (English Channel) palaeoriver represents the largest river system that drained the European continent during Pleistocene lowstands, particularly during the Elsterian (Marine Isotope Stage 12) and Saalian Drenthe (MIS 6) glaciations when extensive ice advances into the North Sea area forced the central European rivers to flow southwards to the Bay of Biscay (NE Atlantic). Numerous long piston cores and high-resolution acoustic data from the northern Bay of Biscay have been correlated using a well-constrained chronostratigraphical framework, and for the first time allow the estimation of the discharge of the Fleuve Manche palaeoriver at the end of the last glacial period (i.e. Weichselian). Our results, discussed in the light of relationships between river loads and drainage area characteristics, strongly suggest that the Bay of Biscay collected sediment-laden meltwater from the Northern European lowlands and the southern part of the Fennoscandian ice sheet at the end of the last glacial period and particularly during the paroxysmal phase of the European deglaciation ca 20–18 ka ago. This demonstrates that the Fleuve Manche drainage area at that time was equivalent to those dating from the major Elsterian and Saalian Drenthe glaciations. Based on our data, we propose a revision of the palaeogeographical and palaeoglaciological evolution of the North Sea area while showing that the British and Fennoscandian ice sheets entered the North Sea basin until ca 18 ka initiating isostatic adjustment of the basin. This allowed the North Sea fluvial system to flow southwards through the Dover Strait. Finally, we discuss the Fleuve Manche freshwater discharge during the maximum European ice sheet retreat ca 18 ka. The estimated discharge at this time appears important and sufficiently sustained to possibly contribute to the profound destabilisation of the thermohaline circulation recognised in the North Atlantic from ca 18 to 16 ka.
(Earth and Planetary Science Letters. vol. 290, n° 0012-821X, pp. 459-473, 20/02/2010)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, GM, IFREMER, LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, CLIM, LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, LDO, INSU - CNRS, UBO EPE, CNRS, UEB, CAM
Assessing coastal benthic macrofauna community condition using best professional judgement - Developing consensus across North America and Europe
Benthic indices are typically developed independently by habitat, making their incorporation into large geographic scale assessments potentially problematic because of scaling inequities. A potential solution is to establish common scaling using expert best professional judgment (BPJ). To test if experts from dif- ferent geographies agree on condition assessment, sixteen experts from four regions in USA and Europe were provided species-abundance data for twelve sites per region. They ranked samples from best to worst condition and classified samples into four condition (quality) categories. Site rankings were highly correlated among experts, regardless of whether they were assessing samples from their home region. There was also good agreement on condition category, though agreement was better for samples at extremes of the disturbance gradient. The absence of regional bias suggests that expert judgment is a via- ble means for establishing a uniform scale to calibrate indices consistently across geographic regions.
(Marine Pollution Bulletin. vol. 60, n° 0025-326X, pp. 589-600, 18/02/2010)
SCCWRP, M2C, UNICAEN, NU, INSU - CNRS, UNIROUEN, NU, CNRS, IRSNB / RBINS, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, UOC, GU, CEAB, CSIC
Comments on ``The Depth-Dependent Current and Wave Interaction Equations: A Revision
Equations for the wave-averaged three-dimensional momentum equations have been published in this journal. It appears that these equations are not consistent with the known depth-integrated momentum balance, especially over a sloping bottom. These equations should thus be considered with caution as they can produce erroneous flows, in particular outside of the surf zone. It is suggested that the inconsistency in the equations may arise from the different averaging operators applied to the different terms of the momentum equation. It is concluded that other forms of the momentum equations, expressed in terms of the quasi-Eulerian velocity, are better suited for three dimensional modelling of wave-current interactions.
(18/02/2010)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, SHOM, LOS, IFREMER
New insights into the size distribution of fluorescent dissolved organic matter in estuarine waters
The study of the fate and composition of organic matter in aquatic systems is of interest since organic matter is believed to play a key role in biogeochemical cycles. Fluorescent dissolved organic material (FDOM) represents a major fraction of dissolved organic matter (DOM) but is under explored, particularly in estuarine environments. In this study, 3D fluorescence excitation-emission matrix (EEM) spectroscopy was combined with tangential ultrafiltration (UF) to investigate the changes in FDOM properties during the mixing of fluvial and marine waters in two estuaries on the French Atlantic coast. Water samples were sequentially filtered through three membranes of decreasing molecular weight (MW) cut off points (3 kDa, 1 kDa and 500 Da). Four ultrafiltration fractions were obtained and were analysed using EEM spectroscopy, as well as for their DOC content. Satisfactory fluorescence and DOC mass balances were obtained after the sequential UF fractionation despite the variable DOC concentration and ionic strength of the estuarine samples, showing the reliability of the UF systems. DOC distribution in the different size classes was observed to vary between the upstream and downstream parts of the estuaries and to be different in the two estuaries. The value of combining UF and EEM spectroscopy was shown, since the coupling allowed observation and isolation of some fluorophores which could not be distinguished in the spectra of bulk samples. Thus, the β fluorophore, observed mainly in marine samples until now, and which is associated with material of autochthonous biological origin, was present in the lowest size fraction (MW < 500 Da) from all water samples, including freshwater ones. The γ fluorophore, characteristic of protein-like compounds, was observed in the small size fractions of some fresh and brackish water samples, but it was not visible in the spectra of the corresponding bulk water samples. The differences in fluorescence properties of the size fractions of DOM were further evaluated by determining three fluorescence indices: the fluorescence index (f450/f500), the humification index (HIX) and the index of recent autochthonous contribution (BIX). The origin and degree of transformation of DOM can be assessed through the calculation of these indices. We showed that the most humified OM was associated with different size fractions, depending on water sample origin, and was characterised by intermediate MW (higher for fresh than marine water). We also showed that DOM distribution and characteristics differ between the Gironde and Seine estuaries.
(Organic Geochemistry. vol. 41, n° 0146-6380, pp. 595-610, 08/02/2010)
ISM, UB, INC-CNRS, CNRS, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Mercury methylation rates of biofilm and plankton microorganisms from a hydroelectric reservoir in French Guiana
The Petit-Saut ecosystem is a hydroelectric reservoir covering 365 km2 of flooded tropical forest. This reservoir and the Sinnamary Estuary downstream of the dam are subject to significant mercury methylation. The mercury methylation potential of plankton and biofilm microorganisms/components from different depths in the anoxic reservoir water column and from two different sites along the estuary was assessed. For this, reservoir water and samples of epiphytic biofilms from the trunk of a submerged tree in the anoxic water column and from submerged branches in the estuary were batch-incubated from 1 h to 3 months with a nominal 1000 ng/L spike of Hg(II) chloride enriched in 199Hg. Methylation rates were determined for different reservoir and estuarine communities under natural nutrient (reservoir water, estuary freshwater) and artificial nutrient (culture medium) conditions. Methylation rates in reservoir water incubations were the highest with plankton microorganisms sampled at − 9.5 m depth (0.5%/d) without addition of biofilm components. Mercury methylation rates of incubated biofilm components were strongly enhanced by nutrient addition. The results suggested that plankton microorganisms strongly contribute to the total Hg methylation in the Petit-Saut reservoir and in the Sinnamary Estuary. Moreover, specific methylation efficiencies (%Me199Hgnet/cell) suggested that plankton microorganisms could be more efficient methylating actors than biofilm consortia and that their methylation efficiency may be reduced in the presence of biofilm components. Extrapolation to the reservoir scale of the experimentally determined preliminary methylation efficiencies suggested that plankton microorganisms in the anoxic water column could produce up to 27 mol MeHg/year. Taking into account that (i) demethylation probably occurs in the reservoir and (ii) that the presence of biofilm components may limit the methylation efficiency of plankton microorganisms, this result is highly consistent with the annual net MeHg production estimated from mass balances (8.1 mol MeHg/year, Muresan et al., 2008a).
(Science of the Total Environment. vol. 408, n° 0048-9697, pp. 1338 - 1348, 01/02/2010)
LCPME, INC-CNRS, UL, CNRS, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Does disturbance drive the collapse of biotic interactions at the severe end of a diversity–biomass gradient?
It has been recently proposed that the decrease in diversity towards the severe end of the humped-back diversity–biomass model of Grime was driven by a collapse of facilitation due to extreme conditions of either stress or physical disturbance. In order to test the hypothesis that disturbance is the primary direct factor driving the collapse of interactions occurring along environmental severity gradients, we conducted a removal experiment in the highly stressed French coastal dunes along a gradient of disturbance due to sand burial. Four dune species were used as targets and transplanted with and without neighbours in four communities along the gradient. The experiment was conducted twice, a dry and an average year. Results of the experiment showed that during the dry year the effect of the environment was prominent and only one species was facilitated for survival in the least disturbed community. During the average year, interactions for growth were important only in the same community, with positive or negative responses depending on the natural position of the target species within the coastal dune gradient. In accordance with our hypothesis, most interactions for both survival and growth were observed in the least disturbed community exhibiting the highest diversity. There were no interactions in the most disturbed community with the lowest diversity.
(Plant Ecology. vol. 206, n° 1385-0237, pp. 287-295, 01/02/2010)
ECODIV, UNIROUEN, NU, BioGeCo, INRA, UB, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS